Data acquisition and control systems, as well as microprocessor and microcontroller based boards and units, typically include “digital” inputs and outputs. There are many types of digital inputs and outputs, and their interface capabilities can differ depending upon the manufacturer's specification. A digital input typically consists of a power supply (voltage source), a switch and a voltage-sensing device (analog-to-digital converter).
Depending on the switch's open/closed status, the sensing device detects a voltage or no voltage condition, which in turn generates a logical 0 or 1, on or off, alarm or normal or similarly defined state. A digital output typically consists of a switch (either mechanical as in a relay, or electronic as in a transistor or triac) that either opens or closes the circuit between two terminals depending on the binary state of the output.
The most common type of digital input is a contact closure. Essentially a sensor or switch of some type closes or opens a set of contacts in accordance with some process change. An applied electrical signal then determines whether the circuit is open or closed. Current flows if the circuit is closed, registering, for example, a “1” in a transistor at the computer interface. Conversely, an open circuit retains a high voltage (and no current), registering a “0” at the transistor.
Accordingly, there are several types of digital inputs and outputs, which are commonly included in many control systems and board products and units when used for specific applications. For example, low voltage, direct current (DC) wetted contacts are common digital inputs to control systems used in relay applications.
A relay is usually an electromechanical device that is actuated by an electrical current. The current flowing in one circuit causes the opening or closing of another circuit. Relays are like remote control switches and are used in many applications because of their relative simplicity, long life, and proven high reliability.
Over the years, a full range of relay products from highly specialized relays from communication equipment to general-purpose relays have been designed to control nearly every function in commercial and industrial processes used in everything from household appliances to industrial machinery. In the home, relays are used in refrigerators, washing machines and dishwashers, and heating and air-conditioning controls.
A power generation plant is one example of an industrial process in which a large number of electrical contacts (e.g., relays and switches) are used. The electrical contacts, such as the relays, in a power generation plant can be used to control a wide variety of equipment such as motors, pumps, solenoids and lights. Highly sophisticated relays can be utilized to protect electric power systems against trouble and power blackouts as well as to regulate and control the generation and distribution of power.
In the example scenario described above, a control system needs to monitor the relays within the power plant to determine their status in order to ensure that certain functions associated with the process are being performed. Often, wetted contacts are utilized to monitor the status of the relays. A relay circuit works by using a sensing unit, the electric coil, which is powered by alternating current or direct current.
When the applied current exceeds a threshold value, the coil then activates the armature, which either closes the open contacts or opens the closed contacts. When power is supplied to the coil, it produces a magnetic force that activates the switch mechanism. The magnetic force passes along the action from a circuit to another. The first circuit is called the control circuit; the second is called the load circuit.
The contacts are the most important constituent of a relay. Their characteristics are significantly affected by factors such as the material of the contacts, voltage and current values applied to them (especially, the voltage and current waveforms when energizing and de-energizing the contacts), the type of load, operating frequency, and bounce. If any of these factors fail to satisfy a predetermined value, problems such as metal degradation between contacts, contact welding, wear, or a rapid increase in the contact resistance may occur.
Contact input status detection circuits are used to detect the status of relay contacts when in use in the field. The contact input status detection circuit monitors provide an indication of potentially degraded electrical relay performance due to contamination.
In industrial environments, contamination routinely interferes with the operation of the relay's contact. Contaminants, which can include oxidation films or foreign particles, tend to produce contact resistance readings that are either high or unstable. Contamination commonly happens with low current applications, usage in high temperature and humidity environments, and during extended periods of storage.
For example, in small currents and low voltage applications, oxidation of relay contact is simply a buildup of corrosion on relay contact surfaces over a period of time. The contacts develop oxidation, which is a thin layer of oxide on the contact surface. It causes problems by increasing the resistance across the contacts which, depending on the amplitude of the voltage being switched, can cause loss of signal or overheating of the contacts.
Oxidation on relay contacts is especially a problem with small currents and low voltages, because they cannot punch through the oxide layer once it accumulates and becomes too thick. One conventional approach to resolve this issue for small currents and low current applications is by passing the required wetting current through the relay contacts so that it punches through the oxide layer. The wetting current is the minimum current needed to flow through a contact to break through any film (contact oxidation) that may have been deposited on the switch.
When the contact is in an open position, the circuit is broken and no current may flow. When closed, the low voltage power supply will provide current which flows through the contact and the sensing circuit. Thus, the circuit must have low enough resistance to allow a substantial current to flow, approximately a few milliamperes (mA), to improve reliability by minimizing oxidation of the contacts and to improve contact wiping.
Thus, another common digital input to control systems, which is often used in relay applications, is “current sink.” The current sink can be considered a load with a special behavior, in which it will increase its own impedance when the voltage across it increases, and decrease it as much as the voltage drops, thus keeping the current through it also constant. A current sink circuit can be used to supply a few mA to a load such as a relay.
It is important to note that most logic gates can “sink” a very limited amount of current, usually in the order of a few mA. Thus, universal input/output (IO) application-specific integrated circuits (ASICs) have a restriction by design. The I/O pins have limited current sink capability so that its current sink is fixed at approximately 7.5 mA. However, when a system needs to support multiple ASIC chips on a board, such as two or three ASICs redundantly, the fixed current sinks add. This generates excessive current and requires active circuitry to be included on dual terminal board or triple modular redundancy (TMR) board configurations.